The present invention relates to a shaped, molded article made of a core of hard metal body and a wear-resistant, corrosion-resistant hard metal surface layer on the core and to a method for producing such an article.
It has long been known that hard metal bodies can be formed from at least one binder or bonding metal of iron, cobalt and nickel and at least one hard metal refractory carbide of at least one of the elements titanium, zirconium, hafnium, vanadium, niobium, tantalum and tungsten. The hard metal body generally is formed by uniting a powdered form of the hard metal carbide by compression with the binding metal, followed by sintering. During the sintering process, the product generally receives its final shape and dimensions and the resulting sintered product is a molded, shaped, hard metal body which often is referred to as a cemented carbide. The hard metal bodies possess great hardness and find wide application in metal turning and cutting tools which are hard enough to permit high turning and cutting speeds in rock or metal.
Increasing demands have been placed on hard metal bodies and there has been a continuing search to provide hard metal bodies having still greater wear resistance. To this end, there has been produced hard metal bodies comprising a core of a shaped, hard metal body formed from a hard metal carbide and bonding metal as described above and a surface coating of a hard material on the core. The surface coating of hard material has been made from such materials as carbides, nitrides, carbonitrides, borides and/or oxides.
Molded hard metal bodies having a core of a hard metal body and a surface coating of a hard material are known to be very hard at the surface and/or have a low tendency to heat-weld. Workpieces made of such surface-coated molded hard metal bodies are therefore very wear-resistant and have high surface hardness. The surface coating of the hard material generally is formed in such a manner that carbides, nitrides, carbonitrides, borides and oxides as well as their mixtures are deposited on the core of the hard metal body during a separate process step. For example, deposition from the gaseous phase according to the chemical vapor deposition process is a preferred method of forming a surface coating on a hard metal body. For example, titanium carbide has been deposited from the gaseous phase to form a hard metal surface coating on a core of a hard metal body.
Tools and other articles made of the known hard metal bodies coated on their surface with a hard material have the primary drawbacks that a complicated procedure is required to produce them and the surface coating often forms a bond of unsatisfactory stability with the hard metal body. In the past, the surface coatings of hard material have had a tendency to come loose from the core of hard metal body in an undesirable manner, and especially when the articles have been subjected to great toughness stress. Further, the use of such articles for turning and cutting operations is possible only within limits because the tools are subjected during this use to high impact stresses and strong alternating thermal stresses which often cause the surface coating of hard material to chip off which leads to premature failure of the tools. Hard surface coatings of a layer thickness of more than 20.mu. have particularly poor adhesion to the underlying core of hard metal body. In practice, this means that only hard surface coatings having a layer thickness of between 5 to 10.mu. can be used. Although the wear-resistance of a hard metal body having a surface coating of a hard material should increase with increasing layer thickness of the surface coating, hard surface coatings with a layer thickness of more than 20.mu. generally cannot be used because under the alternating thermal stresses occurring during use in cutting and turning operations, they come off of their core of hard metal body, before they are worn out, due to lack of adhesion. Attempts have been made to overcome these drawbacks by providing metallic intermediate layers between the core and surface coating or a plurality of hard layers, but these attempts have not been entirely successful.